The present invention relates to an apparatus for monitoring the output from a nuclear reactor used in a nuclear power plant and, more particularly, to a wide range monitor apparatus for the output from a nuclear reactor, which detects changes in neutron flux density nV which are caused by randomly occurring fission and which fall within a wide range of more than ten digits per flux, and which detects the output corresponding to the detection result.
The conventional monitor apparatus for output from a nuclear reactor must be able to monitor output from a nuclear reactor, i.e., the neutron flux density which changes over a wide range of more than ten digits per flux when the reactor is started or stopped. However, since such a monitor range is too wide, measuring over the entire range cannot be performed with a single monitor technique. In view of this, some monitor techniques having different monitor ranges are combined for monitoring the entire range. For example, in a BWR (Boiling Water Reactor) monitor system, the overall monitor range is divided into two ranges. A pulse counting technique is used for one of the two ranges, i.e., the low neutron flux density range. However, a Campbel measuring technique is used for the other of the two ranges, i.e., the high neutron flux density range. Thus, the overall range including the low and high output regions can be monitored.
Even if the neutron flux density changing within a wide range is detected by different measuring techniques, if monitoring can be performed with a single type of monitor apparatus including only one type of detector, the overall system of the nuclear reactor measuring equipment can be simplified. The operation can be facilitated, the running cost can be reduced, and the maintenance procedures can be simplified.
In view of this, there has been proposed a monitor apparatus including only one type of detector. The detector generates a single output signal proportional to a logarithm of a neutron flux density which changes over a wide range requiring the use of two different measuring techniques. An example of such an apparatus is a wide range monitor apparatus as disclosed in Japanese Patent Publication No. 48-18436, entitled, "Random Pulse Monitor Apparatus," corresponding to U.S. Pat. No. 3,579,127.
This random pulse monitor apparatus has the configuration shown in FIG. 1.
High voltage from a DC power source 14 is applied through an impedance element 15 between a pair of electrodes 12 and 13 of a nuclear fission ion chamber 11. The electrode 13 of the fission ion chamber 11 to which the impedance element 15 is connected supplies an ionized signal through a capacitor 16 to the input side of a broad-band amplifier 17. An amplified signal from the broad-band amplifier 17 is supplied to a logarithmic count rate channel 19 and a Campbel channel 20 through a cable 18. The logarithmic count rate channel 19 is for the low neutron flux density range obtained by dividing the entire monitor range into low and high neutron flux density ranges. The logarithmic count rate channel 19 consists of a high-pass filter amplifier 191 and a logarithmic count rate circuit 192. An input pulse signal amplified by the high-pass filter amplifier 191 is converted into a logarithm by the logarithmic count rate circuit 192. A signal is produced from the circuit 192 as an output from one channel which is proportional to the logarithm of the pulse count rate of the incident neutron flux density. The Campbel channel 20 corresponds to the high neutron flux density range and consists of a high-pass fitler amplifier 201, an average rectifier circuit 202, a logarithmic amplifier 203 for generating output proportional to the logarithm, and a differential amplifier 204. An input signal amplified by the high-pass filter amplifier 201 is detected by the average rectifier circuit 202 and is converted by the logarithmic amplifier 203 into a signal proportional to the logarithm of the incident neutron flux. The differential amplifier 204 calculates the difference between the signal from the amplifier 203 with a bias voltage 205 and produces the difference as an output of the other channel. Outputs from the respective channels are supplied to a coupler 21. The coupler 21 adjusts the input signals such that the output signals from the respective channels proportional to the logarithm of the neutron flux density are aligned, couples them and produces a single output. The coupler 21 clamps the output voltage of the logarithmic count rate channel which exceeds a predetermined output voltage at an output voltage corresponding to a predetermined logarithmic count rate (e.g., 10.sup.8 neutron flux density) within a region in a linear region in which the input/output characteristics of the logarithmic count rate channel and those of the Campbel channel overlap. The coupler 21 also adjusts the bias voltage 205 of the differential amplifier 204 of the Campbel channel for cutting off the output voltage below the predetermined logarithmic count rate. A sum of the output from the logarithmic count rate channel and that from the Campbel channel is obtained at a coupling point 211. The coupling point 211 then produces a single output voltage in which the outputs from the logarithmic count rate and Campbel channels are continuously coupled.
The operation of the conventional wide range monitor apparatus of the output of a nuclear reactor having the above configuration will be described below.
The DC component of the output from the nuclear fission ion chamber 11 is cut by the capacitor 16, and the remaining signal component is supplied to the logarithmic count rate channel 19 and the Campbel channel 20. The logarithmic count rate channel 19 produces a signal proportional to the logarithm of the neutron flux density until it reaches the neutron flux density value 10.sup.8 indicated by 22 in FIG. 2. When the density exceeds the value 10.sup.8, the output voltage is decreased due to the influence of the pulse resolution counting loss of the nuclear fission chamber 11. Thus, the higher the neutron flux density value, the greater the decrease in the output voltage, thus providing the characteristics shown by curve A in FIG. 2.
The output from the Campbel channel 20 can be proportional to the logarithm of the neutron flux density value within the range above a predetermined value, for example, the neutron flux density value 10.sup.8 indicated by 22 in FIG. 3. When the density is below 10.sup.8, the output voltage does not become proportional to the neutron flux density due to circuit noise or background radiation noise, thereby providing the characteristics as indicated by curve B in FIG. 2.
The outputs from the logarithmic count rate channel 19 and the Campbel channel 20 are added, and are coupled by the coupler 21 when the neutron flux density exceeds 10.sup.8.The coupled output voltage has the characteristics indicated by line C in FIG. 2.
However, since the output voltages from the logarithmic count rate channel 19 and the Campbel channel 20 are simply added together in the conventional random pulse monitor apparatus as described above, various problems are encountered.
First, when the output from the logarithmic count rate channel 19 is below 10.sup.8, the corresponding output voltage draws closer to the saturation region due to the resolution count loss of the nuclear fission chamber. However, when this output from the channel 19 exceeds 10.sup.8, the corresponding output voltage is decreased. When the output voltage decreases below the clamp level indicated by 23 in FIG. 2, the clamp function cannot be provided. As a result, the output voltage obtained from the coupling point 211 has a portion in which the relationship between the output voltage and the neutron flux density is not linear and is generated since the output from the logarithmic count rate channel is decreased even if the output from the Campbel channel is increased, as indicated by the dotted line C1 in FIG. 4. Thus, the output from the nuclear reactor cannot be correctly monitored.
Second, when the nuclear reactor which has been operating at the rated output stops operating, the neutron flux density abruptly decreases immediately after the stop timing. However, noise is thereafter generated due to residual background .gamma.-rays. Then, the output from the Campbel channel 20 does not decrease below a certain value corresponding to the predetermined neutron flux density, as indicated by curve B1 in FIG. 3. This influence appears in the output from the coupling point 211. The output voltage vs. predetermined neutron flux density characteristic curve does not have the linear relationship as indicated by C2 in FIG. 4 at the coupling point 211 of the outputs from the Campbel channel 20 and the logarithmic count rate channel 19.
Third, when the nuclear reactor is started, the density of the generated neutron flux is increased at a predetermined rate. However, when the rate changes abruptly, the normal operation of the nuclear reactor is prevented. In view of this problem, a protective measure is taken in which the neutron flux density is periodically monitored and the operation of the nuclear reactor is stopped when a detected change in the neutron flux density exceeds a predetermined value. However, in the conventional apparatus, the output voltage corresponding to the critical neutron flux density 10.sup.8 at which the outputs from the logarithmic count rate channel 19 and the Campbel channel 20 are switched is preferably kept constant. However, as shown in FIG. 5, the neutron flux density for limiting the output voltage of the logarithmic count rate channel at a clamp level may take a slightly lower value as indicated by 22" and may deviate from the neutron flux density 10.sup.8 due to the drift of the adjusting point over time or the like. This neutron flux density may also be higher than the density 10.sup.8. Similarly, the neutron flux density for cutting off the output voltage from the Campbel channel 20 may be slightly higher by 22"' and may deviate from the density 10.sup.8, or may also be slightly lower. ln this manner, when a clamp level corresponding to a predetermined neutron flux density deviates due to a drift over time, the output voltage may not be proportional to the logarithm of the neutron flux density at the coupling point 211. In this case, even if the neutron flux density increases, while it is within the range between 22" and 22"', the periodic monitor output indicates that the neutron flux density has not changed even if the rate of change of the output from the nuclear reactor is constant. When the neutron flux density is 22" or 22"' even if the rate of increase in the output from the nuclear reactor stays at a predetermined rate, the periodic monitor output changes as if the neutron flux density has abruptly changed. When such an output deviation occurs, if the output from the nuclear reactor has a region D1 of slow change in the vicinity of a predetermined neutron flux density, the above-mentioned proportional relationship cannot be maintained. Therefore, the output voltage does not precisely follow the change in neutron flux density as indicated by D2 in FIG. 6.